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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
A feature of using a motor generating into a resistor, or a water brake, as your load, is that it won't deliver high torque at low RPM. This generally isn't a problem, as you tend not to use prime movers in that speed/torque region. An IC engine for instance would stall there. If you did want to estimate what an electric motor was doing at low speed, you can generally interpolate between stall and middling speeds with little error. You can of course run a generating motor actively, forcing a current into it, but things then get a lot more complicated. Don't forget the generating motor will get as hot from unavoidable losses as the motoring motor will.
I like Klugesmith's suggestion of a clothes peg, with the grip strengthened as required with elastic bands. It's fast, simple, and sometimes you just need to build and measure something to get an idea of whether you are barking up completely the wrong tree.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
Some sort of stepper motor (I have some) from a really ancient printer dumping into a Schottky rectifier and constant current for each winding might work. Between a range of RPMs it should be pretty accurate once the temperature of the windings and thus copper losses can be calculated, if MPTT is used. The output voltage would thus be (roughly) proportional if RPM is also measured to output energy in joules or dynes.
Also by measuring the temperature rise in the water bath containing the rectifiers and constant current units you can estimate total energy expended as well.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
If you have a suitable LARGE rheostat to hand it may be worth exploring the electrical avenue. If a decent rheostat is an expensive option then maybe the waterbrake option is worth considering?
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Trying to figure out how to make small dynomometer for electric and piston motors.
